The present invention relates to mechanical fluid mixing apparatus and particularly to impeller apparatus and draft tube apparatus for mixing fluids and liquids having particles suspended therein, and for pumping and aerating liquids. Impeller apparatus in accordance with the present invention is particularly suitable for use in draft tube mixing apparatus for wastewater and sewage sludge treatment, as well as in industrial and chemical processes for the treatment and conversion of chemicals and minerals.
Draft tube mixers have been used to provide controlled flow of fluids, including liquid-particle suspensions, in order to effect the mixing thereof. In addition, draft tube mixers, in conjunction with gas introduction means such as air sparging devices, have been in use in order to effect mass transfer from a gas, for example air or oxygen, to the mixed fluid in the mechanical aeration thereof.
Draft tube mixers typically comprise a substantially vertically oriented draft tube, having an axial flow impeller rotatably disposed therein. The draft tube and impeller are typically submerged beneath the surface level of the fluid to be mixed. The impeller is rotated by drive means, for example an electric motor connected to the impeller shaft through a geared or other speed reducing drive, in order to mix the fluid and pump it through and out of the draft tube. For gas to liquid transfer applications, for example aeration of a mixed liquor, an air sparging device may be positioned in the draft tube beneath the impeller. Rotation of the impeller mixes the liquor and pumps it downwardly through the draft tube past the air sparging device thereby creating a flow of aerated mixed liquor. Such apparatus is useful, for example, in biological wastewater treatment systems which require oxygenation and mixing of the liquid medium.
It has been found that axial flow pumping efficiency is detrimentally affected due to turbulence and backflow which may exist between the tips of the impeller blades and the surrounding draft tube. One way of decreasing the turbulence and backflow, thereby increasing the axial flow pumping efficiency in a draft tube apparatus, is to maintain minimal clearance tolerances between the tips of the impeller blades and the interior circumference of the surrounding draft tube. This solution is unsatisfactory due to additional cost of equipment required to achieve and maintain such close tolerances.
It has also been found that the turbulence and backflow contribute to a drastic reduction in the effectiveness of draft tube mixing aerators due to premature flooding of the submerged turbine. That is, the operation of the apparatus reaches a point at which the buoyancy of the air introduced through the air sparging device overcomes the downward pumping of liquid regime promoted by the impeller, thereby severely reducing the mixing and oxygen transfer efficiency of the system.
Conditions which are believed to contribute to flooding are depicted in FIG. 1, which shows a draft tube submerged turbine aerator apparatus, in accordance with the prior art, generally designated 10. The apparatus 10 comprises a draft tube 12 having a cylindrical portion 13 and a conical portion 15. An impeller 14 is rotatably disposed within the draft tube 12 just above the intersection between the conical portion 15 and cylindrical portion 13 of the draft tube 12. The impeller 14 is connected by a vertical shaft 16 to rotating means, for example an electric motor driven geared or other speed reducing drive (not shown). An air sparging device 18 is disposed within the cylindrical portion 13 of the draft tube 12 beneath the impeller 14.
Ideally, rotation of the impeller 14 in the direction shown by arrow A draws fluid into the conical portion 15 and causes the fluid to be pumped axially downward past the air sparging device 18 through the cylindrical portion 13 and out of the bottom end thereof. In actual operation, however, rotation of the impeller 14 in the direction shown by arrow A forces the fluid downwardly and outwardly in directions indicated by arrows 20 and 22 respectively. The outward fluid flow component strikes the inner circumference of the draft tube 12, and more particularly in the embodiment shown in FIG. 1 strikes the canted interior surface of the conical portion 15, causing a portion of this outward flowing fluid to be directed upwardly as indicated generally by arrows 24. This upward or reverse fluid flow creates turbulence as well as backflow in a manner schematically illustrated by arrows 25. This turbulence and backflow reduces the axial flow pumping efficiency of the apparatus.
Introduction of compressed air by the sparging device 18 into the liquid exacerbates the problem of reduced axial flow pumping efficiency by creating an upward, or reverse flow component due to the buoyancy of the air, which the impeller 14 must overcome in order to pump the liquid downwardly through and out of the draft tube 12. As the volume of compressed air increases, the upward force component created by the compressed air increases and acts to combine with the upward component of the liquid flowing between the tips of the impeller blades and the inner circumference of the draft tube 12. This combination continues to exert a resultant upward force which increases as the volume of the compressed air increases. When this resultant upward force prevails over the downward force of the fluid pumped by the impeller 14, the flooding condition occurs.
It has also been found that the composition of the fluid can have a detrimental effect on the operation of the apparatus. In particular, certain foreign objects in wastewater, such as rags, strings, and the like, tend to accumulate on the leading edges of the impeller blades thereby creating a reduction in pumping efficiency and air dispersion capacity; while, at the same time, causing an increase in the power drawn by the electric motor used to rotate the impeller. Although the foreign objects will disengage from the impeller blades when the apparatus is shut down, they will gradually reaccumulate thereon following restart.